1. Field of the Invention
The present invention relates to a method for controlling the rotational speed of a motor-compressor used in an air conditioner for vehicles, and more particularly to a method for controlling the rotational speed of a motor-compressor via an inverter.
2. Description of the Prior Art
There are two types of systems which are used as air conditioners for vehicles. One is a system using a compressor driven by an engine of a vehicle via a belt, etc. The other is a system using a motor-compressor driven by an electric motor. In either type of system, refrigerant is not charged in the refrigerant circuit constituting the air conditioner before the air conditioner is attached to a vehicle. Refrigerant is charged into the refrigerant circuit after the air conditioner is attached to a vehicle and the refrigerant circuit is vacuumed.
When refrigerant is charged, in the case of the system using a compressor driven by an engine, the rotational speed of the compressor can be controlled to an appropriate rotational speed by controlling the rotational speed of the engine. In the case of the system using a motor-compressor, however, since the rotational speed of the compressor is controlled in accordance with the ambient temperature of the vehicle, an atmospheric condition, a set temperature of the air blown into the interior of the vehicle and so forth, the rotational speed of the compressor is not controlled to a constant speed. Therefore, the condition of the refrigerant charge is not stable.
In the system using a compressor driven by an engine, a proper amount of charged refrigerant can be determined by recognizing the amount of charged refrigerant through a sight glass provided in the refrigerant circuit. However, in the system using a motor-compressor, in a case where the system is started under a condition that the temperature of the interior of the vehicle is relatively high and the temperature of the air blown into the interior to be controlled is set to a relatively low temperature, the motor-compressor is driven at a high rotational speed. As a result, there is a concern that the refrigerant may be over charged.
Moreover, at a time immediately after charge of refrigerant is started, the refrigerant is sent into the refrigerant circuit, not by the motor-compressor, but by the pressure difference between the pressure in the refrigerant circuit and the pressure in a bottle of refrigerant so that the pressure in the refrigerant circuit reaches a saturated pressure. Therefore, if the motor-compressor is driven at a high rotational speed under a condition where the amount of refrigerant existing in the refrigerant circuit is small, the compressor portion of the motor-compressor may be damaged. On the contrary, if the motor-compressor is driven at a very low rotational speed or under a condition where the motor-compressor may be stopped from the relationship with various setting temperatures, it becomes impossible to charge refrigerant.